Controlling the flux of photons is crucial in many areas of science and technology. Artificial materials with nano-scale modulation of the refractive index, such as photonic crystals, are able to exercise such control and have opened exciting new possibilities for light manipulation. An interesting alternative to such periodic structures is the class of materials known as quasi-crystals, which offer unique advantages such as richer Fourier spectra. Here we introduce a novel approach for designing such richer Fourier spectra, by using a periodic structure that allows us to control its Fourier components almost at will. Our approach is based on binary gratings, which makes the structures easy to replicate and to tailor towards specific applications. As an example, we show how these structures can be employed to achieve highly efficient broad-band light trapping in thin films that approach the theoretical (Lambertian) limit, a problem of crucial importance for photovoltaics.

The “Epifree” process involves the lift‐off of a high‐quality monocrystalline film formed by reorganization upon annealing of cylindrical macropore arrays in silicon, and can thus provide high‐quality silicon films without resorting to costly epitaxy. The challenge of this new process lies in etching controlled and regular pores in silicon in a cost‐efficient way, and in developing a process compatible with the difficulty of handling a micron‐thin material. Proof‐of‐concept cells have previously been achieved and this paper presents the latest progress, with a first development of thicker films and the inclusion of rear‐side passivation. The energy‐conversion efficiency of 1‐µm‐thin Epifree cells was improved from 2.6 to 4.1% by depositing a stack of amorphous silicon (a‐Si) layers as rear‐side passivation. The increase in Voc was, however, limited and bound to a drop in Jsc. The choice of a‐Si was revealed to be unsuitable because of the thinness of the film and the presence of a full aluminum rear contact. The thinness of the film leads to a decrease in rear‐side reflectivity by the a‐Si absorption, and the aluminum, although not leading to crystallization, partly migrates inside the a‐Si stack upon anodic bonding as shown by TEM. These factors indicate that an alternative surface passivation should be developed. In parallel to process developments, the material was thickened by modifying the macropore array dimensions, leading to a 2.4‐µm‐thick material over 1 cm × 1 cm areas. The efficiency of the next cells is expected to increase with this thicker material. Copyright © 2010 John Wiley & Sons, Ltd.

A concept that could provide a thin monocrystalline-silicon absorber layer without resorting to the expensive step of epitaxy would be very appealing for reducing the cost of solar cells. The empty-space-in-silicon technique by which thin films of silicon can be formed by reorganization of regular arrays of cylindrical voids at high temperature may be such a concept if the high quality of the thin film could be ensured on centimeter-large areas. While previous works mainly investigated the influence of the porous array on the final structure, this work focuses on the practical aspects of the high-temperature step and its application to large areas. An insight into the defects that may form is given and the origin of these defects is discussed, providing recommendations on how to avoid them. Surface roughening, pitting, formation of holes, and silicon pillars could be attributed to the nonuniform reactions between Si, SiO2, and SiO. Hydrogen atmospheres are therefore preferred for reorganization of macroporous arrays. Argon atmospheres, however, may provide high-quality silicon thin films as well, possibly even more easily transferable, as long as annealing is performed in controlled, clean, and oxygen-free conditions. Our experiments on large areas also highlight the importance of kinetics, which had not been considered up to now and which will require further understanding to ensure a complete reorganization over any wafer area.

We report on the fabrication of two-dimensional periodic photonic nanostructures by nanoimprint lithography and dry etching, and their integration into a 1-μm-thin monocrystalline silicon solar cell. Thanks to the periodic nanopatterning, a better in-coupling and trapping of light is achieved, resulting in an absorption enhancement. The proposed light trapping mechanism can be explained as the superposition of a graded index effect and of the diffraction of light inside the photoactive layer. The absorption enhancement is translated into a 23% increase in short-circuit current, as compared to the benchmark cell, resulting in an increase in energy-conversion efficiency.